Laying the Blame for Extreme Weather

For Immediate Release

Boulder, CO, USA - Floods, tornadoes, droughts and wildfires: They are all weather-related, but blaming the latest meteorological disaster on climate change has always been a tricky matter that climate scientists have been shy to do. After all, how can you point to a specific and local event, such as a tornado or dry spell, and say it is caused by something as long-term and huge as global warming?

"That's been the mantra of the community and I think it's wrong," said climate scientist Kevin Trenberth of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. Trenberth and other climate scientists will be giving presentations that connect extreme weather over the past decade to climate change at a session of The Geological Society of America meeting in Minneapolis on Tuesday, 11 October 2011.

The sea surface temperatures near all the extreme flooding events of 2010 were at record levels, Trenberth explains. That includes the Caribbean, Gulf of Mexico, N. Atlantic and the Indian Ocean.

"All of the storms are being formed in an environment that is warmer and wetter than before," said Trenberth. "The main thing that has happened with climate change is that you have changed the environment."

Specifically, the waters are about one degree Fahrenheit warmer than pre-1970 values, leading to air that's four percent wetter. All that additional moisture and heat in the air feeds storms. "That's the climate change kicker. It's the extra nudge that indeed makes you break records."

Another way of looking at it is in terms of the odds of extreme weather events. Extreme weather is always possible, after all. But with warmer oceans, such events are easier to create.

"We're loading the dice in favor of extreme weather events," said Trenberth.

The same goes for droughts and subsequent wild fires. They are the flip-side of the extreme storms in a global atmosphere. While unusually wet monsoons were flooding Pakistan in 2010, the same event helped to block moisture from reaching southern Russia. That led to heat waves and fires. This kind of situation reinforces cyclonic and anticyclonic patterns in the atmosphere which make some areas wetter and others drier and hotter.

"So there are dynamical connections," Trenberth said. "You can't disrupt one part of the atmosphere without getting effects in the whole." And with the history and continued releases of carbon dioxide and other greenhouse gases, plus pollution into the atmosphere, there is no doubt that the system is being disrupted," he said.